1. Field of the Invention
This invention relates to a voltage testing circuit, and more particularly to such a circuit which uses an indicator element such as a lamp or light-emitting diode or the like to give a voltage indication.
2. Description of the Prior Art
In a portable instrument using a battery as the voltage source, it must be known whether or not the voltage supply battery has a normal voltage in order to ensure normal operation of electric circuits or the like used therein. This has often been accomplished by using an ammeter. Such method has proved to be disadvantageous in that the pointer in the ammeter is sometimes difficult to see and that the ammeter is too bulky to incorporate it in a limited space such as a camera or the like. There is another method which employs a lamp, a light-emitting diode or the like adapted to turn on to indicate the normal or abnormal condition of the voltage supply battery. This latter method is highly preferable because of its simplicity and visual advantage, and often resorts to the combination of a single transistor and a voltage divider provided by resistors, the lamp or the like being turned on or off in accordance with the comparison between the base-emitter voltage V.sub.BE of the transistor and the source voltage, However, in such arrangements the indication is readily affected by temperature variations or irregularities in the transistor's base-emitting voltage V.sub. BE and the emitter ground current amplification factor h.sub.FE. Thus, the latter method has been very unsatisfactory in terms of temperature characteristics and accuracy.